1. Field of the Invention
This invention relates to heating of a metal cylindrical element used in the injection molding or extrusion of feed materials such as plastic resins, formable foodstuffs (i.e. pasta) and appropriate metals (i.e. magnesium). Relevant heated cylindrical elements include barrels, feed pipes or adaptor pipes, dies, nozzles, etc. All such elements are typically heated with resistive contact heaters and are used to plasticate the feed materials by some combination of heating, melting, shearing, mixing, metering and conveying, prior to discharging them under pressure through a nozzle or die. The description of the invention herein focuses on its application to a barrel, but the principles, methods and merits of the invention are equally applicable to any heated cylindrical metal element used in the plasticating of feed materials.
2. Description of the Prior Art
Referring now to FIG. 1, solid plastic feed material, typically in the form of pellets or powder, enters the feed end 1 of the barrel 2 and then is sheared, mixed and metered by a screw 100 that rotates within the barrel 2. The resulting molten material is then forced out under pressure through a nozzle or die at the discharge end 3 of the barrel. To help melt the material, the barrel 2 is also heated, conventionally with external resistive contact heaters 4 commonly referred to as band-heaters.
AC induction has also been used to heat cylindrical plasticating elements such as injection molding and extrusion barrels, by inducing eddy currents within the cylinder's wall to produce direct resistive heating of the cylinder or barrel 2. Improved commercialized induction barrel heating systems include a substantial thermal insulating layer between the induction coils and the barrel to increase barrel-heating efficiency and reduce temperature control response time. A suitable such induction barrel heating system is described in U.S. patent application titled “Apparatus and Method for Inductive Heating a Workpiece Using an Interposed Thermal Insulating Layer”, published Jun. 12, 2008 at U.S. Publication No. 2008-0136066,
The band-heater's or induction heating system's electrical circuitry is usually arranged so that the barrel 2 can be heated in multiple controllable zones 5 along its length (typically three to six zones, but fewer or more are possible), with typically one thermocouple 6 located in the barrel wall per zone to provide temperature measurement feedback. The nozzle or die at the discharge end 3 is usually heated and temperature controlled separately using one or more dedicated band-heaters 7.
Referring still to FIG. 1, band-heaters 4 add substantial thermal mass to the barrel 2, increasing temperature control response times and making it more difficult to control processing temperatures, particularly under changing conditions. The controllability of band-heaters is also further diminished, and they are increasingly prone to overheating and premature failure, if they are covered by thermal insulation 8. For these reasons, band-heaters 4 are usually left exposed to ambient, which unfortunately leads to significant heat losses and waste of energy.
Referring next to FIGS. 2A and 2B, recent AC induction barrel heating systems 9 eliminate the thermal inertia of the barrel heating means to improve control response. Induction barrel heating systems 9 typically also incorporate a layer of thermal insulation 10 interposed between the barrel 2 and the external induction coils 11 to eliminate heat losses to ambient. However, in spite of their advantages, induction systems 9 have the drawback of specialized components that can be relatively expensive, including high-frequency power supplies 12, and depending on the application, specialized coil cables 11. Together, these power supplies 12 and induction coils 11 can also incur heat losses of typically between 5% on high-performance systems and 20% on relatively inexpensive, low-quality systems. And, finally, induction systems 9 require specialized power sources (voltage and number of phases) that often differ from those used by the band-heaters they replace.
As described in U.S. Pat. No. 6,285,006 B1 and illustrated in FIG. 3, rollers 13 used on sheet manufacturing and conversion processes, which have typically steel cores 14, can be manufactured with an internal or external laminated ceramic coating 15 (shown applied to the external surface of the roller in FIG. 3) that acts as a resistive heating layer. The laminated coating 15 comprises a first layer of electrical insulating material 16 applied to the inner or outer surface of the core 14 using a suitable method such as plasma spraying. To increase the bond-strength a bonding layer (not shown) can also be previously applied between the core 14 and electrical insulating layer 16.
A ceramic heating layer 17 is then applied on top of the electrical insulating layer 16 by a suitable method such as plasma spraying. A final optional layer or sequence of layers 18 can then be applied over top of the resistive heating layer 17 to provide external electrical insulation, added durability, or a surface sealing function to prevent contamination of the resistive heating layer 17. This final external layer or series of layers can also be applied by a suitable means such as plasma spraying. Electrodes 19 can then be used to connect an external DC or AC power source 20 to the ceramic heater layer 17 in order to generate resistive heating of the heater layer 17 and hence the roll 13.
As further noted in U.S. Pat. No. 6,285,006 B1, various materials can be used for each layer 16, 17, 18 and the layer thicknesses can be adjusted to provide various properties. As cited in U.S. Pat. No. 6,285,006 B1, suitable materials and thicknesses for use on a 75 mm (3 inch) diameter×400 mm (16 inch) long steel cylinder would be:                Optional Bonding layer—100μ (4 mil) Sulzer Metco 480 nickel aluminide bond coat;        Inner electrical insulating layer 16—250μ (10 mil) Saint Gobin 204 stabilized zirconia;        Ceramic resistive heating layer 17—12-25μ (0.5 to 1 mil) Eutectic 25040 titanium dioxide; and        ptional outer electrical insulating layer 18—250μ (10 mil) Saint Gobin 204 stabilized zirconia.        
Referring still to FIG. 3, U.S. Pat. No. 6,285,006 B1 also describes an example in which a 75 mm (3 inch) diameter 21 by 400 mm (16 inch) long 22 steel roller core 14 is coated with the above materials to produce a ceramic heater layer 17 with an electrical resistance of about 29 ohms, resulting in a heat generation rate of about 2000 watts when 240 volts AC is applied across the electrodes 19. With a roller surface area of about 970 cm2 (150 inch2) this equates to a heat generation density of about 2.1 watts/cm2 (13.3 watts/inch2). The roller in this example is then cycled over 800 times from 70° C. (160° F.) to 315° C. (600° F.) without failure, and operated at up to 370° C. (700° F.) before failing.
Referring now to FIGS. 1 and 3, there is little difference between a steel roller core 14 and a steel cylindrical plasticating element such as a barrel 2, so it follows that the functional layers 16, 17 and 18 described in the above example could be applied in the same manner to the external diameter 23 of a plasticating barrel 2 over any length-wise portion 24. Furthermore, it follows from basic electrical engineering principles that as the roller core diameter 21 or barrel diameter 23 are changed, the heat generation density (i.e. watts/cm2) will remain essentially the same (provided the thickness 25 of the heater layer 17, the length of the roller segment 22 or barrel segment 24 between the electrodes 19, and the applied voltage, all remain unchanged). This is because the axial cross-sectional area of the heater layer 17 increases linearly with roller diameter 21 or barrel diameter 23, thereby reducing the electrical resistance of the heater layer 17 inversely with the diameter 21, 23, which in turn linearly increases the dissipated power to maintain a constant heat generation density. It also follows that if the length of the roller segment 22 or barrel segment 24 is changed, the thickness 25 of the heater layer 17 must be inversely changed to maintain a constant heat generation density. In practice, the electrical resistance of the heater layer 17 decreases in a non-linear fashion as its thickness 25 is reduced and this relationship must be taken into account when specifying the thickness 25 needed to achieve a given heat generation density.
The operating temperature of the heated cylindrical plasticating elements (such as barrels) used in the vast majority of injection molding and extrusion applications is below 315° C. (600° F.). In addition, as indicated in Table 1, the required heat generation density of most barrel heating applications remains essentially constant and below about 2.4 watts/cm2 during machine startup and 1.2 watts/cm2 during normal production conditions. Referring again to FIGS. 1 and 3, most barrels 2 also have heater control zones lengths 5 of 200 to 1200 mm (8 to 48 inches), meaning that ceramic heater layer thickness 25 of under 4 mils should be adequate in most cases, provided a ceramic heater layer material is used that has similar properties to that used in the example above.
Furthermore, most injection molding and extrusion operations shut down and start back up only about once per week, equating to only about 50 full-temperature cycles per year, and therefore well under 1,000 cycles over a 15-year machine life. Plasticating barrel applications are also static, unlike the dynamic rotating loads experienced on roller applications. The external surface 26 of plasticating barrels 2 is also not normally exposed to regular wearing contact, nor is the external surface's condition critically important to the proper functioning of the barrel 2.
The laminated ceramic coating 15 applied to rollers 14 as described above should, therefore, be equally applicable to plasticating barrels 2 having typical external diameters 23 and operating at typical processing temperatures.
TABLE 1Typical Plasticating Barrel SpecificationsScrew Diametermm2060100140180inch0.792.363.945.517.09Barrel approx. length (L/D = 19)mm3801140190026603420inch15.044.974.8104.7134.6Typical barrel sell priceUSD13892385321255088124Barrel approx. outside diametermm79154230305380Barrel approx. heated surface areacm29435526137052548140854Nominal number of zonesper barrel34567Barrel approx. mass (incl. screw & resin)kg1516761615213042Band-heaters approximate total rated powerkW3.22358105164Band-heater maximum power on startup%100100100100100kW3.22358105164watts/inch22227272726watts/cm23.44.24.24.14.0Band-heater approx. efficiency on startup%6060606060Band-heater maximum barrel heating rate onkW1.914356399startupwatts/cm22.12.52.52.52.4Band-heater power use during production%3030303030(approx. total)kW1.06.9173149watts/cm21.01.31.31.21.2